As duplexers for high-frequency circuits of wireless terminals such as mobile phones, filters using Surface Acoustic Wave (SAW) resonators or piezoelectric thin film resonators have been widely used. The duplexer includes a plurality of filters such as a transmit filter and a receive filter. As filters such as a transmit filter and a receive filter, used are ladder-type filters including a series resonator and a parallel resonator.
Japanese Patent Application Publication No. 2013-110655 (Patent Document 1) describes that a chip on which resonators that affect the guard-band side skirt characteristic of the passband of the filter are formed is separated from a chip on which resonators that affect the skirt characteristic at the opposite end of the passband from the guard band are formed. The chips are formed by different element technologies. Japanese Patent Application Publication No. 2005-295496 describes that the electromechanical coupling coefficient of a series resonator is made to differ from that of a parallel resonator.
The technique disclosed in Patent Document 1 can make the guard-band side skirt characteristic steeper and improve the temperature stability and widen the band of the filter. To improve the temperature stability, the absolute values of the Temperature Coefficient of Frequency (TCFs) of the resonant frequency and the antiresonant frequency of the resonator are preferably brought close to zero. However, it is difficult to bring the absolute values of the temperature coefficients of the resonant frequencies and the antiresonant frequencies of all resonators close to zero. Thus, a temperature change may deteriorate the reflection characteristic of the filter.